1. Technical Field
The present invention relates to a positioning device using signals from positioning satellites, a control method of the positioning device, a control program of the positioning device, and a computer readable recording medium recording the control program of the positioning device.
2. Related Art
In the past, a positioning system for positioning a current position of a GPS receiver by using a satellite navigation system, such as GPS (Global Positioning System), has been put into practical use.
The GPS receiver receives signals from a plurality of GPS satellites, and obtains a distance (hereinafter, referred to as pseudo-distance) between each GPS satellite and the GPS receiver by a difference (hereinafter, referred to as delay time) between the time at which the signal is sent from each GPS satellite and the time at which the signal arrives at the GPS receiver. And, a positioning calculation of the current position is performed by using satellite orbit information of each GPS satellite put in the signal received from each GPS satellite and the above-described pseudo-distance.
The GPS receiver can perform three-dimensional positioning to calculate latitude, longitude, and altitude of the current position when the receiver can receive the signal from four or more GPS satellites.
And, the GPS receiver can perform two-dimensional positioning to calculate latitude and longitude of the current position when the receiver can receive the signal from three GPS satellites. The GPS receiver assumes the center of the earth as one GPS satellite and assumes a distance from the center of the earth to the current position as the pseudo-distance. And the GPS receiver performs the positioning calculation as in the case of the three-dimensional positioning. Therefore, in the two-dimensional positioning, the GPS receiver is required to hold altitude information of the current position, in advance.
In relation with the above, a technique of performing the two-dimensional positioning by using altitude obtained by map data (for example, JP A-2002-341012), and a technique of performing the two-dimensional positioning by using altitude calculated by using a combination of the GPS satellites with minimum VDOP (Vertical Dilution of Precision) at a previous positioning, or altitude calculated by a previous three-dimensional positioning (for example, JP B-06-75103) have been suggested.
However, to keep map data places a huge burden on storing means, and there is a case in which an error of altitude information obtained from the map data is large.
And, even if altitude calculated by using a combination of GPS satellites with minimum VDOP at a previous positioning is used, there is a case in which an error of altitude is large due to a bad positioning condition (environment in which signal strength is weak, or multi path is large) at the previous positioning. Further, there is a problem that there is a case in which the error of altitude is large due to the bad positioning condition at the previous positioning, even with a method of using altitude calculated by the previous three-dimensional positioning.